The present invention relates to a method and to a device for detecting the error on the frequency of a carrier.
It finds an important application in systems for the coherent demodulation of phase-modulated signals, that is to say, in particular, signals referred to as PSK signals. For example, it can be used in a digital reception device of a radio link or an optical link.
Recovery of the carrier frequency is necessary for the coherent demodulation of signals which are transmitted by phase modulation of the carrier. In analogue reception devices, this recovery is generally carried out either using a frequency-locked loop or using a phase-locked loop.
There is currently an increasing trend towards digital signal processing. When the reception device is a digital device, there are a number of possibilities for recovering the carrier frequency. If the error on the carrier frequency is sufficiently small, the phase recovery circuit can be used for recovering the frequency as well. In a number of other cases, it is necessary to use a specific circuit for estimating and correcting the error on the carrier frequency.
In the prior art, distinction is drawn between two categories of methods for detecting the error on the carrier frequency, depending on whether these methods use looped structures or direct structures (that is to say in open loop).
In the case of direct structures, frequency estimators are generally used. They give an estimate of the frequency error. One drawback of direct structures is that they generally need to perform summations over a large number of samples, so that the variance of the frequency error is small. This entails a high degree of complexity in their implementation.
In the case of looped structures, a frequency error detector is generally used in a frequency-locked loop. In contrast to frequency estimators, frequency error detectors do not give an estimate of the error, but give a function of the error. In the prior art, distinction is drawn between two main types of detectors. A first type uses the principle of maximum likelihood and is based on the spectra of the signals. A second type uses a frequency estimator as the detector and is based on the time values of the difference between two samples.
Known frequency error detectors have the following drawbacks: those of the first aforementioned type lead to large variances in frequency error, which significantly degrade the performance of the phase recovery circuit. Detectors of the second aforementioned type have the following drawback: in order to give satisfactory results, they generally need to perform a prior summation over the differences between two samples, which entails a high degree of complexity in their implementation.